The present invention relates to a tap for monitoring light within an optical fiber, the tap being particularly useful for controlling an amount of power injected into a single mode fiber from a semiconducting laser diode.
In an optical fiber communications system, it is oftentimes important to accurately control an amount of power injected into an optical fiber, especially if analog transmission is desired. When utilizing a laser in such a system, the output thereof varies over time due to environmental variations, such as temperature and humidity changes, and also due to aging of the laser. In addition, components used for packaging the laser and the optical fiber also tend to be somewhat unstable over time which results in an alignment of the optical fiber and the laser changing. Also since an emission angle of power from a laser changes with laser output power, a percentage thereof accepted by the fiber changes. These variables all affect an amount of power injected into the optical fiber from the laser, and such power variations are detrimental when analog modulation is to be used, especially amplitude modulation.
Numerous proposals have been made in the prior art for linearizing an output of the laser. Back facet monitors have been employed for monitoring an amount of light or optical power emitted from a back end of a laser, and this information is used to determine an amount of power emitted from a front end of the laser confronting the optical fiber. This method has the disadvantage that it is incapable of monitoring any power variations due to changes in fiber alignment over time, and also is incapable of monitoring power variations within a core of the optical fiber due to a change in an angular distribution of light emitted from the fiber which changes with laser output power.
U.S. Pat. Nos. 4,165,496 and 4,351,585 each disclose an optical fiber tap located on an optical fiber downstream from a laser, the tap withdrawing part of an optical signal within the fiber, this signal part being used for feedback control for controlling a power output of the laser. According to these references, the signal is tapped by terminating the fiber and creating a beveled end which is disposed in close proximity to a similarly shaped beveled end of a second optical fiber, the first beveled end causing a portion of the optical signal to be reflected off a face thereof and traversely out of the fiber for detection. A remainder of the signal propagates through the second optical fiber. The withdrawn signal part is then used for feed-back control of the laser output. These methods are disadvantageous since they create unacceptably high losses to the signal in the fiber, and they are relatively difficult to assemble and hence expensive to implement in view of the fact that precise beveled faces of fibers need to be formed and joined by a suitable nonindex matching medium. Finally, cladding modes are detected which introduce erroneous detected signal magnitudes, and hence acceptable feedback control at even low and modestly high speeds is quite difficult.